Large crystal structure distortion in DyB6 studied by X-ray diffraction

Article abstract of DOI:10.1016/S0304-8853(02)01133-2

A large magnetoelastic coupling induces a lattice distortion of a rhombohedral type in DyB₆. Below the quadrupolar ordering temperature T_Q=31K the cubic and the distorted phases coexist. The cubic phase disappears at 23K, which was identified as the magnetic ordering point T_N. In the magnetic phase the distortion (dl/l)₁₁₁ increases monotonically with decreasing temperature and reaches the value of 9 × 10^-3 at 8 K, which is one of the largest value known for cubic compounds.

Full text of DOI:10.1016/S0304-8853(02)01133-2

Journal of Magnetism and Magnetic Materials 258–259 (2003) 529–531
Large crystal structure distortion in DyB studied by X-ray
6
diffraction
S.A. Granovsky*, A.S. Markosyan
Faculty of Physics, M.V. Lomonosov Moscow State University, 119992 Moscow GSP-2, Russia
Abstract
A large magnetoelastic coupling induces a lattice distortion of a rhombohedral type in DyB
6
. Below the quadrupolar
ordering temperature TQ ¼ 31 K the cubic and the distorted phases coexist. The cubic phase disappears at 23 K, which
was identified as the magnetic ordering point TN: In the magnetic phase the distortion ðdl=lÞ111 increases monotonically
À3
with decreasing temperature and reaches the value of 9 Â 10 at 8 K, which is one of the largest value known for cubic
compounds.
r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Rare-earth hexaborides; Quadrupolar ordering; Magnetoelastic coupling
1
. Introduction
rhombohedral crystal structure distortion has recently
been reported for HoB (TQ ¼ 6:1 K, TN ¼ 5:6 K [7]).
TbB has a complicated magnetic phase diagram [8].
6
A large variety of interesting physical properties is
observed in RB hexaborides with the CaCl-type cubic
6
GdB shows a first-order transition at TN ¼ 15 K, and
6
6
Ã
structure. CeB
to the Kondo-effect on the Ce-ions [1], SmB
6
is studied for more than two decades due
is claimed
another magnetic transition occurs at T ¼ 8 K [9]. The
nature of these transitions in this system with zero
6
to be an intermediate-valence compound showing a
semiconductor behaviour at low temperatures. Recently,
it has been reported that the alkaline-earth hexaboride
orbital moment still remains unknown.
DyB6 is an antiferromagnet with TN ¼ 26K. A
second transition above TN associated with ordering of
3
+
CaB
6
doped by small amounts of La shows a weak
the quadrupolar moments of Dy
has been found by
itinerant ferromagnetic behaviour with the ordering
temperature of about 600 K [2]. Due to the low work
specific heat and magnetic susceptibility measurements
at TQ ¼ 32 K [10]. A large crystal distortion and a
complicated magnetic structure have been observed by
neutron diffraction [11]. The goal of the current study is
to investigate the details of the distorted phase in the
6
function, LaB attracts much interest as a perspective
material for thermo-ionic emission [3].
In the domain of multipolar interactions study, CeB
6
is an important object, since it is an archetype of a
system showing antiferroquadrupolar ordering between
TQ ¼ 3:3 K and TN ¼ 2:4 K. However, the mechanisms
causing this ordering are to be studied more in detail
6
quadrupolar and magnetic phases in DyB .
2
. Experimental part
Polycrystals of the rare-earth hexaborides have been
[
6
4,5]. The heavy rare-earth hexaborides RB exist with
R=Gd, Tb, Dy and Ho [6]. Owing to a number of
problems with their synthesis, these compounds are less
investigated. Quadrupolar ordering accompanied by a
obtained by melting the elemental components (99.9%
purity for rare earth and 99% for ‘‘crystallic’’ boron) in
an arc furnace under high-purity argon gas. A massive
copper plate has been used as a bottom. Since all the
*Corresponding author. Tel.: +7-095-939-1145; fax: +7-
0
0
95-939-5907.
E-mail address: ser@plms.phys.msu.su (S.A. Granovsky).
heavy RB
ingots obtained from the stoichiometric starting mixture
6
crystallize by a peritectic reaction, the as-cast
304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 1 3 3 - 2

5
30
S.A. Granovsky, A.S. Markosyan / Journal of Magnetism and Magnetic Materials 258–259 (2003) 529–531
a
and RB12 diffractometer with 2 kW X-ray tubes using Fe-K or
contained considerable amounts of RB
4
,
which have been found to be impossible to remove by
homogenization in a standard furnace with the anneal-
ing temperature up to 12001C. The amount of the
impurity phases was minimized by a suitable tuning of
the starting compositions and melting conditions.
Compounds with R=Y, Tb, Dy and Ho have been
synthesized and their quality has been controlled by X-
ray diffraction. The X-ray patterns of the synthesized
hexaborides are shown in Fig. 1. The positions of the
Co-K radiation. A flow cryostat (Oxford Instruments,
a
CF-100) was used for cooling the sample down to 8 K,
the temperature was controlled with an accuracy of
0.25 K.
6
For DyB the (1 1 0), (2 2 0), (3 2 0) and (4 0 0)
reflections were studied in the temperature range 8–
35 K. With decreasing temperature, the (1 1 0), (2 2 0)
and (3 2 0) reflections split gradually into doublets with
equal intensities. No splitting was observed for the
(4 0 0) reflection in the whole temperature range. The
4
Bragg peaks for the RB (P4/mmm crystallic structure)
and RB12 (Fm3 m) phases are indicated by arrows. The
total amount of the impurity phases has been estimated
as 15% in HoB , 7% in DyB , 4% in YB and no
data obtained show that
structure is stabilized in DyB
a
rhombohedral crystal
at low temperatures. In
6
6
6
6
Fig. 2, the (1 1 0) reflection is shown at some character-
istic temperatures as an example. The patterns at
intermediate temperatures are complicated and can be
interpreted as consisting of non-distorted cubic and
distorted rhombohedral phases. The concentration of
these phases varies with temperature.
foreign phases were found in TbB . Thus, this method
6
has been found to be suitable for preparing samples for
powder diffraction studies.
The temperature dependent X-ray measurements have
been performed on
a
Rigaku-Denki ‘‘Geigerflex’’
Fig. 3 shows the temperature dependencies of the
intensity of the X-ray peaks for the cubic and
RB12
RB12
RB12
RB4
RB4
RB4
RB4 RB4
RB12 RB12
(
100)
DyB6
α 1
(
110)
(110)
TbB6
111)
(
α 2
(
100)
(
110)
YB6
(
111)
35 K
(
100)
(
110)
24.5 K
DyB6
111)
(
23 K
(
100)
21.5 K
(
110)
HoB6
(
111)
45
10 K
20
25
30
35
40
50
38.5
38.6
38.7
38.8
38.9
39
39.1
39.2
39.3
39.4
39.5
2θ
, deg.
2θ, deg.
Fig. 1. X-ray diffraction patterns of the synthesized hexabor-
ides (Co-K radiation). The positions of the Bragg peaks for the
RB and RB12 phases are indicated by arrows in the top of the
figure.
Fig. 2. X ray patterns of the (1 1 0) reflection of DyB
different temperatures (Fe-K radiation). Asymmetry of the X-
6
at
a
a
4
ray peak arises from non-resolved a1 þ a2 doublet as indicated
at T ¼ 35 K.

S.A. Granovsky, A.S. Markosyan / Journal of Magnetism and Magnetic Materials 258–259 (2003) 529–531
531
À3
E9 Â 10 at 8 K. This value is among the largest one
known for cubic compounds: ðDl=lÞ1 E4:5 Â 10À3 for
11
À3
À3
90.6
90.4
90.2
TbFe
NpFe
series a large value of ðDl=lÞ111 (5 Â 10 ) has been found
for HoB , too [7]. Our X-ray measurements have shown
and TbCo
, 7.0 Â 10 for US and 8.0 Â 10 for
2
2
2
[12]. Note, in the heavy rare-earth hexaboride
TQ
À3
6
À4
TN
6
that the lattice distortions in TbB do not exceed 10 .
90
Acknowledgements
3
6
9 12 15 18 21 24 27 30 33
T, K
This work was supported by RFBR, Project No. 00-
2-17844, 02-02-06022.
0
-
cubic phase
References
1] K. Winzer, W. Flesh, J. Phys. Colloid 39 (C6) (1978) 832.
-
rhombohedral
phase
[
6
9
12
15
18
21
24
27
30
33
[2] D.P. Young, D. Hall, M.E. Torelli, Z. Fisk, J.L. Sarrao,
J.D. Thomson, H.-R. Ott, S.B. Oseroff, R.G. Goodrich, R.
Zysler, Nature 397 (1999) 412.
T, K
Fig. 3. Temperature dependence of the intensity of the X-ray
peaks for the cubic and rhombohedral phases of DyB . The
[3] R. Shimizu, T. Shinike, S. Ichimura, S. Kawai, T. Tahaka,
J. Vac. Sci. Technol. 15 (1978) 922.
6
inset shows the temperature dependence of the calculated
rhombohedral angle. The quadrupolar and magnetic transition
temperatures are indicated by arrows.
[
4] J.M. Effantin, J. Rossat-Mignot, P. Burlet, H. Bartolin, S.
Kunii, T. Kasuya, J. Magn. Magn. Mater. 47–48 (1985)
1
45.
[
[
5] G. Iumin, Phys. Rev B 55 (1997) 8267.
6] K.E. Spear, in: Boron and Refractory Borides, Springer,
Berlin, 1977, p. 439.
rhombohedral phases of DyB . They correspond to the
6
content of these phases in the material. As can be seen
from this figure, the quadrupolar ordering temperature
TQ is about 31 K (in good agreement with [10,11]). The
magnetic ordering temperature TN; at which the non-
distorted phase disappears completely, is estimated as
[
7] A. D o. nni, S. Kunii, L. Keller, P. Fischer, T. Herrmanns-
d o. rfer, V. Pomjakushin, PSI Experental Report Book,
2000, p. 49.
[8] M. Amara, R.M. Gal e! ra, S.A. Granovsky, S. Kunii, J.
Phys: Condens. Matter 13 (2001) 6307.
[
9] R.M. Gal e! ra, P. Morin, S. Kunii, T. Kasuya, J. Magn.
2
3 K (26K according to the specific heat data [10]). The
inset in Fig. 3 shows the temperature variation of the
rhombohedral angle, a; of DyB . The jump-like appear-
Magn. Mater. 104–107 (1992) 1336.
[
[
10] K. Segawa, A. Tomita, K. Iwashita, M. Kasaya, T. Suzuki,
S. Kunii, J. Magn. Magn. Mater. 104–107 (1992) 1233.
11] K. Takahashi, H. Nojiri, K. Ohoyama, M. Ohashi, Y.
Yamaguchi, S. Kunii, M. Motokawa, Physica B 241–243
6
ance of the lattice distortion proves that the transition at
TQ is of a first-order type. This angle is weakly
temperature dependent in the quadrupolar range and
corresponds to ðDl=lÞ111E6 Â 10À3: Below TN; this
parameter increases monotonically and reaches
(
1998) 696.
[12] K.P. Belov, G.I. Katayev, R.Z. Levitin, S.A. Nikitin, V.I.
Sokolov, Usp. Fiz. Nauk 140 (1983) 271.